The present invention relates to power steering gears for motor vehicles and, more specifically, to cooling systems for such power steering gears.
Motor vehicles often employ power steering assist systems to reduce the amount of effort needed for steering the vehicle. Such a system generally employs a power steering pump that pumps a fluid, such as a hydraulic fluid, with a high pressure. The pumping pressure is used to assist the steering effort of the vehicle. The power steering pump generates heat when in use, which heats the fluid. This heat can reduce the lifetime of the steering gear due to higher wear and leakage on power steering gears. The steering gear is also often positioned close to the engine that also generates heat. On modern trucks, the engine is encapsulated in order to reduce noise, which in turn increases the temperature around the engine. In order to cool the steering gear oil, it is known to install a heat exchange system in the high pressure part of the system. Known systems for such cooling include air-cooled heat exchangers in which the pump fluid is cooled by air in a manner similar to the liquid cooling that takes place in a radiator for the engine coolant. However, such a cooling system is very dependent on the airflow that may be very limited at low or at idle speed. Furthermore the cooling effect may also be insufficient when the temperature of the surroundings is high. There is also a disadvantage in installing a heat exchanger in the high pressure part, as the installation is expensive due to the high pressure. A leakage in the high pressure part will also be more severe than in the low pressure part.
To increase the cooling rate, the systems may use the engine coolant from the engine. An example of such a system is known from US 2003116104A, in which a cooling jacket is provided about a periphery of the power steering pump housing for cooling the pump housing. This cooling jacket cools the steering pump, but is relatively large and increases the arrangement complexity.
EP 1219526 A and EP 968902 A shows hydraulic fluid reservoirs with an integrated heat exchanger for cooling the fluid. The heat exchanger is connected to the engine coolant.
U.S. Pat. No. 5,513,490 A and U.S. Pat. No. 5,718,281 A shows metallic hydraulic fluid reservoirs with integrated cooling fins for cooling the fluid. This solution is dependent on the surrounding air temperature.
U.S. Pat. No. 5,002,117 A shows a system where the return tube is much longer than necessary, creating a cooling circuit. This solution creates a high loss in the return line, which may require a larger pump to make up for the losses.
All the proposed solutions may work in some cases, bur they all displays different drawbacks, mainly by increasing the system weight, cost and arrangement complexity.
It is desirable to provide a compact cooling system for cooling of the power steering fluid in a motor vehicle. It is also desirable to provide a cooling system that can be used to modify already mounted power steering systems to provide cooling of the power steering fluid.
The present invention relates in a first aspect to a power steering gear for a heavy vehicle, said power steering gear comprising a housing, a flow path for a power steering fluid, and a heat exchanger mounted to said housing or being a part thereof, said heat exchanger comprising a first flow path for the power steering fluid and a second flow path for a coolant adapted to exchange heat with the power steering fluid.
Since the heat exchanger is a separate unit, it can either be integrated in the design of new power steering gears or used to modify operational systems already in use when a problem with insufficient cooling of the power steering fluid arises. The present invention thereby provides a relatively cheap and flexible solution. The option of mounting the cooling system to already mounted power steering systems furthermore renders it possible to adjust new vehicles to the customers' specific needs at the time of delivery.
The power steering gear comprises a high-pressure fluid inlet part receiving the power steering fluid from the pump and a low-pressure fluid outlet part returning the power steering fluid to a reservoir or the pump, and the heat exchanger may be mounted adjacent to or on to said inlet or outlet part. Preferably, the heat exchanger is mounted to the outlet part, thus cooling the fluid on the low-pressure side of the system.
Alternatively, the heat exchanger can be an integrated part of said inlet or outlet part such that when assembling the power steering gear, the outlet or inlet part comprises the heat exchanger which then provides an even more compact design of a power steering gear with cooling.
The heat exchanger preferably comprise a plate cooler with an inlet and outlet for the coolant, and the coolant may be water supplied to the heat exchanger by a specific pump for that purpose, but preferably the coolant is an engine coolant of an engine of the vehicle to which the power steering is mounted. Thus, the inlet and outlet of the heat exchanger is coupled directly to the coolant system of the engine of the vehicle, such that the coolant is cooled by the engine radiator.
The cooling system may further comprise air-cooling means for providing an even better cooling of the power steering fluid.
According to a second aspect, the invention relates to a cooling system for a power steering gear, said system comprising a heat exchanger adapted to be mounted to the housing of a power steering gear or being a part thereof and comprising a flow path for a coolant adapted to exchange heat with the power steering fluid.
The heat exchanger may be adapted to be mounted to an already operational power steering gear, or it can be an integrated part of the power steering gear.
The heat exchanger may comprise means for mounting it adjacent to or on to a power steering fluid inlet or outlet of a power steering gear. The heat exchanger may comprise an adapter plate allowing mounting of the heat exchanger to different kinds of power steering gears having different inlet or outlet pipe stub or other parts protruding from its housing. Thus, the same type of heat exchanger can be mounted to different kinds of power steering gear by means of the adapter plate.
The heat exchanger preferably comprises a plate cooler with an inlet and outlet for the coolant, and as mentioned above, it preferably uses an engine coolant from an engine of a vehicle to which the power steering gear is mounted.
The cooling system may further comprise air-cooling means for providing an even better cooling of the power steering fluid.
According to a third aspect, the present invention relates to a heavy vehicle comprising a power steering gear according to the first aspect and comprising a cooling system according to the third aspect.
According to a fourth aspect, the present invention relates to a method of cooling the power steering fluid of a power steering gear on a heavy vehicle, the method comprising the steps of:                providing a heat exchanger on to a power fluid inlet or outlet of the power steering gear,        pumping a coolant through the heat exchanger for exchanging heat with the power fluid so as to cool the power fluid.        
The step of pumping is preferably provided by a coolant pump of the engine of the vehicle, said coolant being cooled by the engine radiator.
It is an advantage of the use of a cooling system in which the pump fluid is cooled by the engine coolant that it is possible to increase the pump flow without risking to overheat the system. Such an increase in the flow may e.g. be advantageous at idle speed. It is a further advantage of the present invention that it can be employed without any need to change the present system and pump design. It is a further advantage of the present invention that the power steering fluid may be led through the heat exchanger via e.g. pipes having relatively small wall thickness. This makes the cooling more efficient than when the system is based on cooling of other parts of the pumps, such as the whole pump housing.